Many biologically active drugs possess strong lipophilic properties, which result in negligible solubility in water. The development of an appropriate formulation of these active substances therefore is a challenging problem.
One alternative to currently available formulations of these pharmacologically active substances is the encapsulation in biodegradable polymeric nano- and microparticles. Such depot formulations for hydrophobic drugs or peptides and proteins are widely known and described in the literature.
However, most all prior art processes use halogenated hydrocarbons as solvent (dichloromethane or chloroform), which have enormous toxicological potential (Henschler. D; Agnew. Chem. 196 (1994), 1997-2012). Examples of prior art processes are discussed briefly below.
I. Solvent Evaporation
Solvent evaporation involves the dissolving of the polymer in an organic solvent which contains either dissolved or dispersed active agent. The polymer/active agent mixture is then added to an agitated continuous phase which is typically aqueous. Emulsifiers are included in the aqueous phase to stabilize the oil-in-water emulsion. The organic solvent is then evaporated over a period of several hours or more, thereby deposing the polymer around the core material. The solvent evaporation procedure is disclosed in U.S. Pat. No. 4,389,330.
However, the solvent evaporation technique is often not preferred because active ingredient is often lost during the solvent extraction process. This is because the process involves emulsification into an aqueous phase, and a water soluble drug will often rapidly partition from the more hydrophobic polymer-solution phase into the aqueous surroundings.
Encapsulation by the solvent evaporation process also leads to the production of microspheres. The active ingredient to be encapsulated is traditionally dispersed in a solution of polymer in a volatile organic solvent. This phase is emulsified by means of a surface-active agent in a non-miscible dispersing medium (water or mineral oil). The organic solvent evaporates with stirring. After the evaporation, the microspheres are recovered by filtration or centrifugation.
The advantages of the technique are the absence of toxic solvents such as heptane, and the absence of agglomeration of the microspheres. Solvent evaporation is simpler, more flexible and easier to industrialize than other processes such as phase separation or coacervation, and it makes it possible to use reduced amounts of solvent.
Traditionally, solvent evaporation is primarily applied to the encapsulation of lipophilic substances such as steroids and nitrosoureas. The microencapsulation of hydrophilic active ingredients requires the use of an apolar dispersing phase such as a mineral oil. Acetone/paraffin systems are conventionally used. However, the levels of incorporation of the hydrophilic active ingredient into the microspheres relative to the amounts employed in the process are fairly low and, moreover, this system involves a limitation with respect to the types of polymers which may be used given that it requires the polymer to be soluble in acetone, which is the case with lactic acid polymers, but which is not the case for lactic acid and glycolic acid copolymers. This technique by emulsion/evaporation is therefore traditionally recognized as unsuitable for water-soluble peptides and for all water-soluble substances.
Microparticles produced according to the solvent evaporation method are described in two Canadian Patent Applications, CA 2,100,925 (Rhone-Merieux) and CA 2,099,941 (Tanabe Seiyaku Co.).
According to CA 2,099,941, the water soluble active ingredient and the biodegradable polymer are initially dissolved in a solvent or a solvent mixture. The solvent/solvent mixture is then eliminated and the formed solid dispersion dissolved in another organic solvent immiscible with water. The resulting solution (oil phase) is emulsified in an aqueous phase so that a O/W emulsion is formed. The organic solvent of the oil phase is finally evaporated. Specific examples cited in the patent describe the use of poly-(lactide-co-glycolide) polymer (PLGA) as matrix and thyreotropin releasing hormone (TRH) or one of its derivatives as active principal.
The components are initially dissolved in a mixture of acetonitrile/ethanol and optionally water, or only acetonitrile, or in a mixture consisting of acetonitrile and aqueous gelatin or dichloromethane and ethanol.
Organic solvents, like dichloromethane or chloroform, are used to dissolve the forming solid dispersion. An aqueous polyvinyl alcohol solution represents the aqueous phase. The size of the microparticles lies at a diameter from 1 to 100 μm and, according to the specific examples, at about 50 μm to ≦100 μm.
According to CA 2,100,925, microparticles of LHRH hormone and analogs are produced by dispersal of the powdered LHRH hormone in two organic solvents, the one solvent (dispersion solvent) permitting production of a homogeneous suspension by simple agitation. The second solvent is readily miscible with water and therefore makes microdispersion of the organic phase in the aqueous phase possible. Dichloromethane or, as an alternative, chloroform is used as second solvent. The microparticles have a diameter of 1-250 μm. The microparticles are preferably larger than 50-60 μm.
The morphology of the microparticles so produced is again very nonhomogeneous. As already mentioned above, the employed halogenated solvents are also toxicologically objectionable. This method also requires large amounts of surfactants.
II. Phase Separation
Another technique which can be used to form microparticles in phase separation which involves the formation of a water-in-oil emulsion or oil in water emulsion. The polymer is precipitated from the continuous phase onto the active agent by a change in temperature, pH, ionic strength or the addition of precipitants. Again, this process suffers primarily from loss of active ingredient due to denaturation.
Consequently, the use of phase separation for production of microparticles may be better suited for the formulation of microparticles containing more water soluble compounds, particularly water-soluble polypeptides. Phase separation methods of microparticle preparation allow a more efficient incorporation of drugs and can easily be scaled up for industrial purposes. The process of phase separation usually employs an emulsion or a suspension of the drug particles in a solution of a high molecular weight polymer and an organic polymer solvent. A non-solvent is then added to the suspension or emulsion, causing the polymer to separate from solution and to encapsulate the suspended drug particles or droplets containing them. The resulting microparticles (which are still swollen with solvent) are then normally hardened by a further addition of a non-solvent or by some other process which strengthens and improves the properties of the microparticles.
First, the product to be encapsulated is dispersed in the solution of a polymer intended to subsequently form the matrix of the microcapsules. Secondly, the coacervation of the polymer is induced by a physico-chemical modification of the reaction medium, in particular by means of a phase separation inducing agent. Thirdly, coacervate droplets that form around the material to be encapsulated are stabilized and solidified by means of a nonsolvent of the polymer, for example heptance.
Pharmaceutical formulations of water-soluble peptides and proteins in microcapsule form that were produced based on coacervation and emulsion phase separation are known from U.S. Pat. Nos. 4,675,189, 4,675,800, 4,835,139, 4,732,763, and 4,897,268; U.K. Patent application No. 2,234,896; and EP 330,180 and EP 0 302 582 and by Ruiz et al. in the international Journal of Pharmaceutics (1989) 49:69-77 and in Pharmaceutical Research (1990) 9:928-934.
Methods are described in these disclosures in which the employed copolymer, preferably poly-(lactide-co-glycolide) polymer, is dissolved in a halogenated organic solvent, preferably dichloromethane, and an aqueous peptide solution dispersed in this polymer solution. A so-called coacervation agent is then added. The coacervation agent is soluble in the employed organic solvent, but the polymer is not, so that precipitation of the polymer occurs with incorporation of the dispersed polypeptides.
Silicone oil is ordinarily used as coacervation agent for phase separation. After addition of silicone oil, a large amount of heptane must also be added, which produces curing of the microcapsules. The encapsulation efficiency of this method is about 70% (U.S. Pat. No. 4,835,139). The microcapsules so produced have a diameter of 1-500 μm, according to the examples preferably 10-50 μm.
The main disadvantage of this method is the use of large amounts of solvents with, in addition to cost constraints, problems of toxicity linked to the solvents, such as heptane, used. This is because the techniques by coacervation using heptane do not enable its complete removal. A large amount of residual solvents, of the order of 5 to 10% of heptane, is observed in the microspheres.
Independently of the above, it has also been observed that aggregates of microspheres causing a high loss of yield in the production of these microspheres by this method and sometimes requiring the total rejection of some batches which have thus become unusable, were often produced. The tendency of the microspheres to aggregate causes additional difficulties at the time of suspending the microspheres for injection, in the case of injectable microspheres.
Another disadvantage of the technique by phase separation is the nonhomogeneous distribution of the active substance in the microspheres with irregular release, and in general a first phase of accelerated release (“burst effect”). This is observed in particular when the active substance is suspended in the polymer solution, in particular because it is not soluble in the solvent for the polymer. This generally applies, for example, to polypeptides. Additionally, problems include the formation of non-spherical particles, formation of particles that are not smooth and have defects, the presence of large particles with a wide range of sizes, and the presence of non-particulate material.
III. Double Emulsion
Another example of a process to form microparticles is shown in U.S. Pat. No. 3,523,906. In this process a material to be encapsulated is emulsified in a solution of a polymeric material in a solvent which is immiscible with water and then the emulsion is emulsified in an aqueous solution containing a hydrophilic colloid. Solvent removal from the microcapsules is then accomplished in a single step by evaporation and the product is obtained.
The double emulsion (W/O/W) and solvent evaporation method, is also disclosed in U.S. Pat. No. 3,523,906 if for technical applications, and employs non-biodegradable polymers as wall material (for example, polystyrene), which are dissolved in halogenated hydrocarbons (dichloromethane or chloroform).
U.S. Pat. No. 5,330,767 describes the use of the W/O/W double emulsion and solvent evaporation method disclosed in U.S. Pat. No. 3,523,906 for pharmaceutical purposes. In contrast to the method described in U.S. Pat. No. 3,523,906, only biodegradable polymers are used here. Other double emulsion process for microencapsulation are disclosed in EP 190,833 and WO 99/58112, and U.S. Pat. Nos. 5,648,095, 5,902,834, 4,954,298, 5,841,451, 4,917,893 and 4,652,441.
A serious shortcoming of these methods, however, is that the microparticles so produced consist of a mixture of monolithic microspheres and microcapsules. In addition to the limited encapsulation efficiency (30-60%), the nonhomogeneous morphology of the microparticles has a significance effect on the release behavior of the product (R. Baker, Controlled Release of Biologically Active Agents, A Wiley-Interscience Publications, 1987). This simultaneously also hampers reproducibility of product quality.
Moreover, the process involves a complex multistep process, in which the specific effect of individual process steps on product quality is uncertain, for which reason process optimization is also difficult. The process is very time-intensive and requires large volumes of surfactant solutions. Another shortcoming of the process is the use of solvents with high toxicological potential (Henschler D. Angew. Chem. 106 (1994), 1997-2012).
IV. Spray Drying
Another method for production of biodegradable microparticles, in which water-soluble peptides and proteins can be incorporated, described in EP 0 315 875 (Hoechst AG), is based on the spray-drying process. In this process, an aqueous peptide or protein solution is emulsified in an organic polymer solution and this emulsion is then spray-dried. Examples of other spray drying processes are disclosed in U.S. Pat. Nos. 5,648,096, 5,723,269, and 5,622,657.
A mixture of polyhydroxybuteric acid and poly(lactide co-glycolide) polymer in a mixing ratio between 99:1 and 20:80 is used as biodegradable polymer. The peptide/protein is then in micronized form or in aqueous solution. Chloroform, dichloromethane, DMF or a solvent mixture of water/ethanol/chloroform are considered are solvent. Chloroform is used in the mentioned examples. Spray-drying preferably occurs at temperatures from 45° C. to 95° C.
Shortcomings of this method include the low yield (45% of the theoretically possible) and the high initial burst effect. In addition, use of solvents, like dichloromethane and chloroform, leads to toxicologically objectionable residual solvent contamination in the end product. Spray-dried microparticles, in principle, also exhibit a strong tendency toward agglomeration, and agglomerates with a diameter of up to 100 μm often form.
In spray drying the polymer and the drug are mixed together in a solvent for the polymer. The solvent is then evaporated by spraying the solution into a drying chamber which is also provided with a source of a drying agent. This results in polymeric droplets containing the drug. However, sensitive substances such as proteins can be inactivated during the process due to the elevated temperatures used and the exposure to organic solvent/air interfaces. Further disadvantages include generation of high porosity due to rapid removal of the organic solvent. A variation that has been introduced to avoid these shortcomings is the use of low temperature during microsphere formation (U.S. Pat. No. 5,019,400, WO 90/13780 and U.S. Pat. No. 4,166,800). Microcapsules have been prepared using spray coating of drug-containing microparticles with PLGA polymers as described in U.S. Pat. No. 4,568,559.
Other examples of microencapsulation methods are known in the prior art. For example, another example of a conventional prior art microencapsulation process is shown in U.S. Pat. No. 3,737,337 wherein a solution of a wall or shell forming polymeric material in a solvent is prepared. The solvent is only partially soluble in water. A solid or core material is dissolved or dispersed in the polymer containing solution and thereafter in a single step, the core material containing solution is dispersed in an aqueous liquid which is immiscible with the organic solvent in order to remove solvent from the microcapsules. In still another process as shown in U.S. Pat. No. 3,691,090 organic solvent is evaporated from a dispersion of microcapsules in an aqueous medium in a single step, preferably under reduced pressure. Similarly, the disclosure of U.S. Pat. No. 3,891,570 shows a method in which solvent from a dispersion of microcapsules in polyhydric alcohol medium is evaporated from the microcapsules by the application of heat or by bringing the microcapsules under reduced pressure. Another example of a one-step solvent removal process is shown in U.S. Pat. No. 3,960,757.
WO 97/19676 discloses a process for microencapsulation of hydrophilic active agents. An aqueous active agent solution having a pH of 6.0-8.0 is added to a polymer solution. An aqueous surfactant phase is then added to form microcapsules comprising an inner aqueous core containing the active agent.
WO 99/20253 discloses a process for forming microparticles wherein a drug emulsion or dispersion is injected into an aqueous polyethylene glycol (PEG) solution which acts as a continuation phase and as an extraction medium. The solvent for the emulsion or dispersion should be immiscible or essentially immiscible but slightly or very slightly soluble in the water/PEG solution. Examples include ethyl acetate, dichloromethane, methyl ethyl ketone and methyl isobutyl ketone alone or in combination. A high concentration of PEG is used to prevent diffusion of active agent from the droplets/particles. The process requires several hours of mixing to produce the microparticles.
Additional process for producing microparticles are disclosed in U.S. Pat. Nos. 6,291,013, 5,792,477, 5,643,605, 5,992,357, 6,309,569 and in PCT publications WO 99/59548 and WO 01/28591. Whatever the process, the drug release pattern for a microparticle is dependent upon numerous factors. For example, the type of drug encapsulated and the form in which it is present (i.e. liquid or powder) may affect the drugs release pattern. Another factor which may affect the drug release pattern is the type of polymer used to encapsulate the drug. Other factors affecting the drug release pattern include the drug loading, the manner of distribution in the polymer, the particle size and the particle shape. Despite numerous modifications to the above processes to produce microparticles for pharmaceutical applications, problems remain which reduce the effectiveness and reproducibility of the microparticles produced by these methods, particularly for use in controlled release delivery systems.